1. Field of the Invention
The present invention relates generally to the field of transmitting digital data in cable television network systems using cable modems. More specifically, it relates to methods and apparatus for identifying efficient upstream channel transitions for reducing upstream frequency noise.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable TV systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable headend, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the headend was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers long. Branching off of these trunk lines were distribution or feeder cables (40% of the system""s cable footage) to specific neighborhoods, and drop cables (45% of the system""s cable footage) to homes receiving cable television. Amplifiers are provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the headend of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 50 to 800 MHz. Broadcast signals were sent downstream; that is, from the headend of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the headend, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the headend of the cable system and distribution nodes (discussed in greater detail with respect to FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers long, whereas a typical coaxial trunk line is about 10 kilometers long , as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of television signals to subscribers. Thus, there needed to be one-way transmission paths from a central location, known as the headend, to each subscriber""s home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 100 to 2,000 subscribers. Although networks exclusively using fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1 is a block diagram of a two-way HFC cable system utilizing a cable modem for data transmission. It shows a headend 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Headend 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. Headend 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108.
Each headend can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the headend and each distribution node. In addition, because cable modems were not used, the headend of pre-HFC cable systems did not contain a CMTS. Each of the fiber nodes 108 is connected by a coaxial cable 110 to duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers, depending on the bandwidth. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio data, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 56,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds of 10 million bps, or more. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable TV system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable TV industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics t o connect headends to fiber nodes and, in some instances, using them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and have a wider bandwidth than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers degrade the signal quality and are susceptible to high maintenance costs. Thus, coaxial distribution systems that use fiber optics have much less need for amplifiers. In addition, amplifiers are typically not needed for fiber optic lines (item 106 of FIG. 1) connecting the headend to the fiber nodes.
In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert a modulated RF signal to digital data and converts the digital data back to a modulated RF signal. The conversion is done at two points: at the subscriber""s home by a cable modem and at a CMTS located at the headend. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal. Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cable commuting. Not surprisingly, with the growing interest in receiving data over cable network systems, there has been increased focus on performance, reliability, and improved maintenance of such systems. In sum, cable companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as full service providers of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
A problem common to all upstream data transmission on cable systems, i.e. transmissions from the cable modem in the home back to the headend, is ingress noise which lowers the signal-to-noise ratio, also referred to as carrier-to-noise ratio, of an upstream channel. Ingress noise can result from numerous internal and external sources. Sources of noise internal to the cable system may include cable television network equipment, subscriber terminals (televisions, VCRs, cable modems, etc.), intermodular signals resulting from corroded cable termini, and core connections. Significant sources of noise external to the cable system include home appliances, welding machines, automobile ignition systems, and radio broadcasts, e.g. citizen band and ham radio transmissions. All of these ingress noise sources enter the cable system over the coaxial cable line, which acts essentially as a long antenna. Ultimately, when cable systems are entirely optical fiber, ingress noise will be a far less significant problem. However, until that time, ingress noise is and will continue to be a problem with upstream transmissions.
The portion of bandwidth reserved for upstream signals is normally in the 5 to 42 MHz range. Some of this frequency band may be allocated for set-top boxes, pay-per-view, and other services provided over the cable system. Thus, a cable modem may only be entitled to some fraction or xe2x80x9csub-bandxe2x80x9d such as between 200 KHz to 3.2 MHz. This sub-band is referred to as its xe2x80x9calloted band slicexe2x80x9d of the entire upstream frequency range (5 to 42 MHz). This portion of the spectrumxe2x80x94from 5 to 42 MHzxe2x80x94is particularly subject to ingress noise and other types of interference. Thus, cable systems offering two-way data services must be designed to operate given these conditions.
As noted above, ingress noise, typically narrow band, is a general noise pattern found in cable systems. Upstream channel noise resulting from ingress noise adversely impacts upstream data transmission by reducing data throughput and interrupting service, thereby adversely affecting performance and efficient maintenance.
When a particular contiguous group of sub-bands or alloted band slices, referred to as an upstream frequency channel or frequency channel, reaches an unacceptable signal to noise ratio, the CMTS begins searching for a cleaner, unused frequency channel for the upstream signal. A frequency channel is used by a group of cable modems (the grouping typically based on physical location) to transmit signals upstream to the headend. A spectrum analyzer located in the headend (discussed in FIG. 2) identifies another upstream frequency channel that has a low power level which indicates that there is little noise on that upstream frequency channel. If a frequency channel has a high power measurement, the channel is very likely already being used by another upstream frequency channel. Conversely, if the spectrum analyzer expects a particular frequency channel to not be transmitting a signal, any power measurement for that frequency channel is a measurement of noise in that frequency channel. Cable modems in the cable plant can be divided into groups (or subscriber areas) in which cable modems in each group share the same upstream frequency channel. This is possible by using, for example, time division multiplexing, a technique known in the art in which each cable modem transmits at a particular time when no other cable modem is allowed to transmit signals.
Using a Fast Fourier Transform (FFT), the spectrum analyzer can measure power levels of the upstream channel and identify an upstream frequency channel that has a low noise level, i.e. a clean frequency channel. Frequency channels having a low power measurement are very likely not being used to transmit a signal, i.e. data, (otherwise they would have a significantly higher power measurement). Thus, any power measurement in those channels is an indication of noise in those channels. The spectrum analyzer can then instruct the headend to change the upstream frequency channel for a group of cable modems to the frequency channel having a lower noise level. However, this transition to another upstream frequency channel may not result in any significant improvement or any improvement at all. If there is a major source of noise in the external or internal environments to the cable plant spanning a wide frequency spectrum, the headend can continually switch upstream frequency channels and still not result in any significant improvement in signal to noise ratio. In this situation and in other less extreme situations it may be better to continue using the currently used frequency channel even if its noise level is above a certain threshold level.
The overhead in traffic on the cable plant resulting from the CMTS having to inform each cable modem to change frequencies can be high. For example, the CMTS must send information to each cable modem indicating on which upstream frequency to send information. This starts an initialization process thereby causing delays in data communications. It is better to avoid using the fiber and coaxial lines for signalling frequency changes and indicating telemetry status of the cable modems. With current systems and techniques, the spectrum analyzer does not measure the noise level of the presently used upstream frequency channel to determine whether transitioning to another frequency is worth the processing overhead. With current systems, the spectrum analyzer simply determines that another frequency channel may have a slightly lower noise level and will instruct the CMTS to switch to another upstream frequency.
Therefore, it would be desirable to be able to measure the noise level of the upstream frequency channel currently in use by one or more cable modems in the cable plant before switching to another frequency channel. This noise-level data can be used to make a more intelligent decision as to whether to transition to another upstream frequency channel or remain on the one currently in use, thereby reducing unnecessary signalling traffic and processing in the headend and at the cable modem.
To achieve the foregoing, and in accordance with the purpose of the present invention, methods and systems for changing upstream frequency channels when the frequency channel presently in use has too high a noise level and, in the process, avoid making frequency channel changes that will not result in a significant improvement are disclosed. In one aspect of the present invention, a method of transitioning from a current frequency channel to another frequency channel in a cable plant is described. A spectrum analyzer examines the power level of the current frequency channel. A bit error rate for the current frequency channel is detected for the frequency channel, having a noise level, by an upstream receiver. The spectrum analyzer then determines whether the bit error rate exceeds a threshold value. If so, the spectrum analyzer then determines whether another frequency channel has a noise level less than the noise level of the current frequency channel by at least a threshold amount. If another frequency channel has such a noise level, the headend changes upstream frequency channels from the current frequency channel to the other frequency channel having the lower noise level.
In another embodiment of the present invention, the noise level of the present frequency channel is measured while the frequency channel is being used to transmit data upstream. The spectrum analyzer can measure the noise level of the current frequency channel and use this data to decide whether to transition to another frequency channel. By comparing the noise level of the presently used frequency channel to other potentially better frequency channels, a spectrum analyzer can make a more informed decision as to whether to transition to another frequency channel. In another embodiment, the spectrum analyzer accumulates noise-level data relating to the present frequency channel at predetermined time intervals, such as during timing marks in which the spectrum analyzer receives an empty data packet sent from an appropriately instructed cable modem. In yet another embodiment, it is determined whether the noise level of the present frequency channel is greater than the noise level of other frequency channels by at least a threshold amount in order to justify transitioning to another frequency channel.
In another aspect of the present invention, a method for measuring noise levels in an upstream frequency channel while the upstream frequency channel is being used to transmit data upstream is described. An upstream frequency channel to be monitored is chosen from among multiple upstream frequency channels in the upstream band. A cable modem using the chosen upstream frequency channel is assigned a timing mark by the headend in which the cable modem can send data upstream to the CMTS. The cable modem is instructed to send an empty data packet to the headend on the chosen upstream frequency channel at the timing mark. Once the empty data packet is received, the noise level of the frequency channel can be measured using the empty data packet.
In another aspect of the present invention, an apparatus in a cable television plant for deciding when to transition between upstream frequency channels and initiating such a transition is described. The apparatus, referred to as a spectrum analyzer, contains a frequency channel noise detector that measures noise levels of multiple frequency channels in an upstream band including that of the frequency channel presently in use having a present noise level. The frequency channel noise detector measures the noise level of the frequency channel in use while the frequency channel transmits data upstream. The apparatus also contains a frequency channel noise level scanner that locates an xe2x80x9calternativexe2x80x9d or preferred frequency channel from multiple frequency channels having a different noise level. The current frequency channel is used for transmitting data upstream if the present noise level is less than the noise level of the preferred frequency channel plus a predetermined noise value.
In one embodiment, the processor coupled or functioning with the spectrum analyzer performs as a data correlator for correlating power level data related to each of the multiple frequency channels. In another embodiment, the processor performs as a comparator for comparing the data error rate of the frequency channel in use against a predetermined threshold data error rate. In another embodiment, the processor associated with the spectrum analyzer performs as an an output device that instructs a downstream transmitter to direct cable modems to use a preferred frequency channel.